2.4.1. What is the no da or n da ending?

No da or no desu after a verb, i adjective, or na is
often used to emphasize the previous statement. For example

atsui n da 暑いんだ, "It's hot."

or

Isha na no da 医 者なのだ, "He's a doctor."

This might be used for explaining,
emphasis, or even insistence. N in n da is a contraction of
no.

Another usage of no is in the phrase XXX no da,
meaning "you should XXX". This is used both in positive (iku n da!, "You should go", or just "Go!"), and in the negative
(fuzakeru n ja nai!, "you shouldn't joke around", or just
"stop joking around!"). This no is used very commonly in
speech, and is very often said as n.